This invention relates to motion picture projectors and more particularly to such projectors which are adapted for analysis of relative motion or other relationships between elements in the pictures.
In many diverse fields, motion pictures are projected at speeds many times slower than they were originally photographed to analyze motion and action or to pinpoint a particular frame or photograph which best illustrates a relationship between elements in the pictures. Such applications include but are not limited to medicine, sports, product testing, surveillance, reconnaissance, time and motion study and motion picture editing.
In all of these applications the viewing situation is similar. A large volume of photographic material is available for review which contains relatively small areas of particular interest to the viewer. The locations of these areas are unknown to the viewer who must be able to efficiently scan the volume of material, locate portions of interest and intensively review certain sections over and over.
When a viewer is engaged in intensive review of a small portion of the material, the content and the changing elements of the individual photographs or frames are of primary importance. Anything which introduces extraneous motion or the appearance of motion degrades the ability of the viewer to make the analytical judgments required in the motion analysis. The optimum image presentation is one in which the viewer can move a single frame or picture in either direction at will without detection of anything but the motion of the subject material between the frames in question.
Most projectors employed for the purposes described above employ a rotating shutter which interrupts the light path a given number of times per second. The projectors attempt to advance a frame during the off or blocking position of the shutter. One problem with many of such projectors is a failure to confine the advancing of the film to the precise period of time when the shutter is fully closed. Any movement of the frame to be exposed during the open portion of the shutter cycle is perceived by the viewer and can appear to be image movement which severely handicaps the viewer's ability to analyze motion.
Another problem with many existing projectors is the inability to precisely position one frame after another at substantially the same spot in a light path. This may result in vertical and horizontal motion of the frame image and appear to the viewer as subject motion.
Other problems occur in changing direction of film movement which may result in shifting of the image or frame motion because of "free play" in the frame positioning apparatus between forward and reverse drive mechanisms.